The present invention relates to a traction distribution control system for a four-wheel drive (4WD) vehicle, which distributes traction of a power source to a pair of main driving wheels or one of pairs of front wheels and rear wheels and a pair of driven wheels or another pair of wheels.
Typically, a traction distribution control system for a 4WD vehicle comprises engaging means arranged in a path transferring power of an engine to the main driving wheels and for transferring power to the driven wheels when the engaging means are in engagement, wherein the engagement state of the engaging means is controlled to control traction distribution to the driven wheels.
This traction distribution control system is constructed such that a difference is obtained between an average speed of the front wheels and an average speed of the rear wheels, wherein as the difference is greater, engaging force of the engaging means is increased to enhance the ratio of traction distribution to the driven wheels, and wherein as the difference is smaller, engaging force of the engaging means is decreased to lower the ratio of traction distribution to the driven wheels. Therefore, when torque slip occurs in the main driving wheels, the ratio of traction distribution to the driven wheels is increased to enhance the vehicle running-ability and stability while reducing torque slip of the main driving wheel.
With the above traction distribution control system, the average speeds of the front wheels and the rear wheels are obtained, respectively, to determine the ratio of traction distribution to the driven wheels, i.e. engaging force of the engaging means, in accordance with a difference between the average speeds. Thus, when the vehicle runs on a road having different road-surface friction coefficients (hereafter refer to friction coefficient as “μ”) for right and left wheels, i.e. a split-μ road, for example, and when one of the main driving wheels is on the low μ road surface and causes a slip, but another main driving wheel does not cause a slip, an average speed of the right and left wheels, which serves as a reference for control, has slower increase than the speed of the slip wheel, and less increasing amount. As a result, the ratio of traction distribution to the driven wheels determined in accordance with a difference between the average speed of the front wheels and the average speed of the rear wheels becomes smaller than an optimum value for stable vehicle running with slip of the main driving wheel restrained. This renders not only slipping of the main driving wheel uncontrollable, but also enhancement in vehicle start-ability and stability difficult.
Moreover, the above problem due to slipping of only one of the main driving wheels can occur not only when the vehicle runs on the split-μ road, but also when the cornering inner wheel has smaller load during cornering.